Secretion of GIT Lecture by Dr Sandeep 9:30 – 10:30 am

Salivary secretion Saliva is produced by the salivary gland at the rate of 1L/day The saliva helps in initial digestion of starches and lipids by salivary enzymes, dilution and buffering of ingested foods, and lubrication of ingested food with mucus to aid smooth movement

Formation of saliva Saliva is an aqueous solution whose volume is high compared to the size Saliva composed of water electrolytes, alpha amylase, lingual lipase, kallikrein, and mucus Compared to plasma it is hypotonic (i.e., has lower osmolarity)

The acinar cells secrete the initial saliva, which is isotonic and has same electrolyte composition The ductal cells modify the initial saliva. The transport mechanisms involved are Luminal: Na + -H + exchange, Cl - -Hco - 3 exchange, and H + -K + exchange The basolateral side contains Na + -K + ATPase and Cl - channels are present Combined action of these transporters provides absorption of Na and Cl and secretion of K + & HCO - 3 takes place

Effect of flow rate on composition of saliva

Regulation of Salivary secretion Salivary secretion is exclusively under neural control by autonomic nervous system Salivary secretion is increased by both parasympathetic and sympathetic stimulation, although parasympathetic stimulation is dominant The parasympathetic to salivary gland is through facial N. (CN VII) and glossopharyngeal (CNIX) Sympathetic innervation originates from T1 –T3

Pancreatic secretion The exocrine pancreas secretes approximately 1L/day The secretion consist of aqueous and enzymatic component

Formation of pancreatic secretion Eznymatic component of pancreatic secretion occurs from acinar cells Most of the enzymes required for digestion of carbohydrates, proteins, and lipids are secreted by the pancreas e.g., pancreatic lipase, amylase, proteases The ER of the acinar cells synthesize and transferred to golgi complex and then to condensing vacoules, where they are concentrated and stored in zymogen granules

The aqueous component is secreted from centroacinar and ductal cells The initial secretion is isotonic and contains all the electrolytes The apical membrane of ductal cells contains Cl - - HCO - 3 exchange and the basolateral membrane contains Na + - K + ATPase and Na/H exchanger

Effect of flow rate on composition of pancreatic secretion

Regulation of pancreatic secretion Aqueous secretion is stimulated by the arival of H + in the duodenum, and the enzymatic secretion is stimulated by products of digestion (small peptides, AA, and fatty acids) Acinar cells (enzymatic secretion) have receptor for CCK and muscarinic receptors for ACh During intestinal phase CCK is the most potent stimulant for enzymatic secretion

The I cells are stimulated to secrete CCK by the presence of AA, small peptides, and fatty acids ACh stimulates enzymatic secretion and potentiates the action of CCK by vagovagal reflex Ductal cells (aqueous secretion) have receptors for CCK, ACh, and secretin, which is secreted by S cells of the intestine The chief stimulant is H + in the lumen of duodenum

Reference Physiology 3 rd edition by Linda S Costanzo Linda S Costanzo